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Date: Wed, 27 Jul 2011 08:52:04 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [V5 Patch 3/4]Qemu: Command "block_set" for
 dynamic block params change
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To: Michael Tokarev <mjt@tls.msk.ru>
Cc: Kevin Wolf <kwolf@redhat.com>, Supriya Kannery <supriyak@linux.vnet.ibm.com>, Christoph Hellwig <hch@lst.de>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@gmail.com>

On 07/27/2011 08:43 AM, Michael Tokarev wrote:
>> Index: qemu/block.c
>> ===================================================================
>> --- qemu.orig/block.c
>> +++ qemu/block.c
>> @@ -651,6 +651,34 @@ unlink_and_fail:
>>       return ret;
>>   }
>>
>> +int bdrv_reopen(BlockDriverState *bs, int bdrv_flags)
>> +{
>> +    BlockDriver *drv = bs->drv;
>> +    int ret = 0, open_flags;
>> +
>> +    /* Quiesce IO for the given block device */
>> +    qemu_aio_flush();
>> +    if (bdrv_flush(bs)) {
>> +        qerror_report(QERR_DATA_SYNC_FAILED, bs->device_name);
>> +        return ret;
>> +    }
>> +    open_flags = bs->open_flags;
>> +    bdrv_close(bs);
>> +
>> +    ret = bdrv_open(bs, bs->filename, bdrv_flags, drv);
>> +    if (ret<  0) {
>> +        /* Reopen failed. Try to open with original flags */
>> +        qerror_report(QERR_REOPEN_FILE_FAILED, bs->filename);
>> +        ret = bdrv_open(bs, bs->filename, open_flags, drv);
>> +        if (ret<  0) {
>> +            /* Reopen failed with orig and modified flags */
>> +            abort();
>> +        }
>
> Can we please avoid this stuff completely?  Just keep the
> old device open still, until you're sure new one is ok.

I may be misremembering, but I thought Christoph had mentioned wanting 
to write a kernel patch to toggle O_DIRECT through fcntl().

This seems like the only way to make this not racy to me.

Regards,

Anthony Liguori